A suspension/solution of certain clays and other additives is used as a mud for drilling. The functions of the mud are to act as a lubricant, a sealant to maintain hole pressure, a coolant for the drill, and a hydraulic carrier of cuttings from the bottom of the hole to the surface. Because of the additives, the mud is normally reclaimed and after reconditioning is re-circulated.
Reclamation has been achieved by removing the drilling cuttings by settlement but current practice is to remove the large cuttings through a coarse mesh (on a shale shaker). The resulting mud containing fine cuttings has then been pumped through hydrocyclones which separate most of the mud from the sand and silt. The removal of the last of the mud from the sand and silt (approximately 5%) has been achieved by passing it through a fine mesh screen. Thus the hydrocyclones handle 100% of the mud (excluding large solids) but only approximately 5% goes to the fine screen.
Hydrocycloning has been found to break up the particles which are to be filtered due to the high shear that occurs in a hydrocyclone. The result is that the particles which are to be filtered may well be broken down to particle sizes equal to that of the mud particles and the result is that there is a minimum size below which particles cannot be removed from the mud. These minute particles equivalent to the size of the mud particles tend to accumulate in the mud until eventually the mud becomes totally unusable since the contaminating particles cannot be separated from the mud particles by filtration.
Clearly therefore, it would be an advantage not to employ hydrocycloning since simple filtration does not break up the contaminating particles into the mud sized particles and 100% filtration can be achieved.
Clearly if hydrocycloning is to be avoided, the whole of the liquid flow must pass through the fine screen. In order to handle the total flow a fine screen would have to be a very large area and further problems arise due to the fact that the fine screen tends to be fragile. These problems are discussed in British Patent Specification No. 1,326,133, reference to which is made for a description of a conventional vibratory screening machine consisting of a vibrating basket inside of which is mounted a woven mesh screen. This Patent Specification described the use of hookstrips for tensioning fine mesh screen. Alternatively this screen may be tensioned during manufacture and glued to a frame ready for quick assembly into the vibrating basket.
The motion of the basket may be linear, elliptical or circular and the mud/solids are fed onto one end of the screen (typically the rear end). The vibration of the screen causes the solids which settle onto the screen to be thrown up and along towards the opposite end of the screen (i.e. the front).
The vibration of the screen serves two purposes:
1. It prevents the solids from accumulating on the screen and blocking the mesh.
2. It conveys the solids to the front of the screen where they can be collected and typically fall off to facilitate disposal.
Typically therefore, the basket is open at the front end to permit the overflow of solids from the screen.
The maximum capacity of a conventional screen constructed along these lines is usually regarded as being when 2/3 of the screen is covered with mud. The remaining 1/3 "dries" the solids. The capacity of such a screen can be increased by increasing the frequency and/or the amplitude of the vibration since this effectively increases the acceleration of the particles and forces more liquid through the mesh. Maximum amplitude and frequencies are limited by basket design and construction.
Conventional screens are mounted either horizontally or sloped downwards towards the front where the particles leave the screen, since this improves the conveying of the particles.
It is, therefore, an object of the present invention to provide an improved vibratory screening machine for the fine screening of liquids particularly for the fine screening of oil well drilling muds.